Method and device for measuring airflows through HVAC grilles

ABSTRACT

A fan powered by a controlled external energy source and a flow measurement device measure the airflow exiting or entering grilles/registers of an HVAC system without impeding the airflow being measured. A flow straightening element is preferably employed to overcome any effects of a non-uniform or swirled airflow profile on the fan. Preferably, the RPM of the fan is measured to determine the airflow. The RPM/flow calibration of the fan is insensitive to pressure differentials or the fan is a pressure-sensitive fan submitted to appropriate pressure differentials. Alternatively, the voltage or current supplied to the external energy source is used to determine the airflow. Finally, the pressure differential across the flow straightening element can be measured to determine airflow, eliminating the impact of pressure differential across the flow measurement device and fan.

[0001] This application claims priority from provisional applicationserial No. 60/358,641 filed Feb. 21, 2002.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to a method and devicefor measuring the airflow exiting the duct system of an HVAC system witha fan powered by a controlled external energy source and a flowmeasurement device.

[0003] Airflow through an HVAC system flows into the system throughreturn grilles or outdoor air intakes, is pushed or pulled by the fan,and then is forced through a duct system, exiting through a supplyregister or grille into a room to provide cooling, heating orventilation. It is often desirable to measure the airflow out of or intothe duct system to determine if the desired flow is being provided to orremoved from each room or zone.

[0004] Airflows entering or exiting grilles are commonly measured usingeither a pressure-measurement grid that attempts to determine averagevelocity or a flow measurement device including an impeller in which thespeed of the impeller is an indication of the speed of the airflow.

[0005] There are several drawbacks to both of these techniques. Pressuremeasurement grids suffer from inaccuracies that are caused by spatiallynon-uniform velocities. Empirical register dependent correction factorshave been employed to account for non-uniform velocity biases withpressure measurement grids, however, the use of these correction factorsis cumbersome and unreliable in the field applications. Flowstraightening elements for pressure-measurement grids have also beenemployed to eliminate the need for correction factors and improveaccuracy, but create excessive flow resistance.

[0006] The process of measuring the airflow with the impeller-based flowmeasurement device also creates flow resistance, impeding the airflowbeing measured. With conventional flow measurement devices, the impelleris spun by energy in the airflow that is being measured, which meansthat the impellear changes the flow being measured. Another problemsometimes associated with the prior art impeller-based flow measurementdevices is that the measurement of the airflow is biased if the airflowexiting the duct system has a swirled profile. Flows have also beenmeasured in research settings using a flow capture hood and a remotecalibrated fan.

[0007] Hence, it would be beneficial to employ a flow measurement devicethat does not impede the airflow being measured and that accommodatesfor non-uniform or swirled airflow profiles.

SUMMARY OF THE INVENTION

[0008] A fan powered by an external energy source and a flow measurementdevice measure the airflow exiting a supply register or grill of an HVACsystem without impeding the airflow being measured. As the fan ispowered by an external energy source rather than being powered by energyin the airflow, there is less flow resistance and impedance to theairflow. Preferably, the fan is insensitive to pressure differentials,or a pressure-sensitive fan is submitted to appropriate pressuredifferentials.

[0009] Because the fan is powered by an external energy source, aflow-straightening element can be employed to overcome any effects of anon-uniform or swirled airflow profile on the fan.

[0010] Preferably, a tachometer is employed to measure the RPM of thefan. As airflow is a function of the RPM of the fan, the RPM of the fanis an indication of the airflow through the supply register or grill.For fans whose RPM/flow calibrations are sensitive to pressuredifferentials, the flow-straightening element is designed to keep thepressure differential across the fan within pressure differentialcalibration limits over the full range of flows measured, so that flowmeasurement is not impacted by a pressure drop or rise across the fan.This is especially important at low RPMs. The pressure drop across thefan should be very low, such as between 0-3 Pa.

[0011] Alternatively, a voltage or current meter is employed to measurethe voltage or current supplied to the external energy source thatdrives the fan, and thereby determine the airflow. As the voltage orcurrent increases, the airflow through the supply register or grilleincreases.

[0012] Finally, a pressure sensor can employed to measure the pressuredifferential across the flow-straightening element and thereforedetermine the airflow. Pressure sensors are positioned in the hood andbetween the straightening element and the fan, respectively, todetermine the pressure differential and therefore the airflow.

[0013] Independent of the chosen technique for sensing the flowrate, acontrol circuit can be employed to automatically adjust the voltage orcurrent supplied to the external energy source to maintain the desiredzero pressure differential between the pressure in the hood and pressurein the room.

[0014] These and other features of the present invention will be bestunderstood from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The various features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows:

[0016]FIG. 1 illustrates a schematic diagram of a top view of the flowmeasurement system of the present invention;

[0017]FIG. 2 illustrates an example flow straightening device;

[0018]FIG. 3 illustrates a graph relating the RPM of the fan to airflowat zero pressure differential across the fan;

[0019]FIG. 4 illustrates a graph relating fan RPM calibrationsensitivity to pressure differential at different airflow rates;

[0020]FIG. 5 illustrates a graph relating airflow to changes in the DCvoltage supplied to the external energy source; and

[0021]FIG. 6 illustrates a graph relating DC voltage supplied to theexternal energy source to airflow for two different types of supplyregisters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022]FIG. 1 schematically illustrates the flow measurement system 20 ofthe present invention. Heated or cooled air from a furnace or airconditioner 22 flows through the duct system 24 and exits through asupply register or grille 26 into a room for heating or cooling. A fan32 powered by an external energy source 30 and a flow measurement device28 a, 28 b or 28 c measures the airflow exiting the duct system 24. Asthe fan 32 is powered by an external energy source 30 rather than beingpowered by energy in the airflow, there is less flow resistance andimpedance to the airflow being measured. Preferably, the RPM/flowcalibration of the fan 32 is insensitive to pressure differentials orthe fan is a pressure-sensitive fan, such as a muffin fan, submitted toappropriate pressure differentials.

[0023] The fan 32 is positioned in front of the supply register orgrille 26 and under a capture hood 36. Because the fan 32 is powered byan external energy source 30, a flow-straightening element 34 ispreferably employed to overcome any effects of a non-uniform or swirledairflow profile on the fan 32. Preferably, the RPM/flow calibration ofthe fan 32 is insensitive to swirl and the flow straightening element 34is not required. If a flow straightening element 34 is employed, theairflow first passes through the flow-straightening element 34 and isstraightened prior to approaching the fan 32. The flow-straighteningelement 34 is a honeycomb, a plurality of straws, or a perforated plate.FIG. 2 illustrates the flow-straightening element 34 including aplurality of straws.

[0024] Several characteristics of the flow measurement system 20 can beused to measure the flow entering or exiting the grill or register 26,including measurement devices 28 a, 28 b or 28 c. Preferably, atachometer 28 a is employed to measure the RPM of the fan 32. As shownin FIG. 3, there is a linear relationship between the RPM of the fan 32and the airflow as long as the pressure drop across the fan 32 is keptbelow 0-3 Pa. Preferably, the pressure drop across the fan 32 ismaintained to prevent a change in the RPM/flow calibration of the fan bymore than 5%. The RPM of the fan 32 is used to determine the airflowthrough the supply register or grille 26 as the airflow is a function ofthe RPM of the fan 32.

[0025] When using the preferred RPM method for measuring airflow forfans 32 whose RPM/flow calibrations are sensitive to pressuredifferentials, such as muffin fans, the flow-straightening element 34must be designed to keep the pressure differential across the fan 32within pressure differential calibration limits over the full range offlows measured, so that the flow measurement is not impacted by apressure drop or rise across the fan 32. This is especially important atlow RPMs.

[0026] The pressure drop across the pressure-sensitive fan 32 should bevery low, such as between 0-3 Pa. Higher pressure drops across the fan32 would influence the RPM-flow calibration of the fan 32, and thereforecomplicate the measurement of the airflow, requiring both RPM andpressure sensing for the calibration. For some pressure sensitive fans32, the RPM-flow calibration only works when the pressure drop or riseacross the fan 32 is kept below 1 Pa at lower flows and below 2-3 Pa athigher flows. The flow range over which a pressure sensitive fans 32 areextra sensitive to the pressure differential across the fans isdetermined by the open area of the fans 32. The impact of pressuredifferential can also be mitigated through fan 32 design. Fans 32 thatare insensitive to pressure differentials are not impacted by thepressure drop or rise across the fan 32.

[0027] An adequately small pressure differential across the fan 32 canalso be maintained by choosing an appropriate diameter for the fan 32relative to the flow resistance of the flow straightening element 34. Aworker skilled in the art would determine the appropriate diameter forthe fan 32 for use in the system 20 to control the pressure differentialby reducing the diameter of the fan 32 until the RPM of the fan 32 ishigh enough that the calibration is not impacted over the full range offlow straightening element 34-presure drop combinations.

[0028]FIG. 4 illustrates the relationship between fan 32 sensitivity topressure differential at different air flow rates and the importance ofkeeping the pressure differential across pressure-sensitive fans 32within small pressure differential calibration limits. As shown, at lowflow rates, such as 53 cubic feet per minute, the calibration of the fan32 decreases as the pressure change increases. At high flow rates, suchas 209 cubic feet per minute, the calibration of the fan 32 staysrelatively constant as the pressure change increases. At all flow rates,the calibration of the fan 32 is constant at about 14 RPM/cfm when thepressure change is zero. Therefore, it is important to maintain thepressure differential across the fan 32 close to zero to maintain aconstant RPM calibration of the fan 32.

[0029] Alternatively, as shown in FIG. 5, a volt or current meter 28 bis employed to measure the voltage or current supplied to the externalenergy source 30 which drives the fan 32 and used to determine theairflow. As the voltage or current increases, the airflow through thesupply register or grille 26 increases. The plot of FIG. 5 was generatedemploying a 10 inch diameter supply register 26 and employing aflow-straightening element 34 of straws and a 8 inch diameter supplyregister 26 employing a flow-straightening element 34 having only halfthe number of straws.

[0030] Finally, a pressure sensor 28 c can be employed to measure thepressure differential across the flow-straightening element 34 (pressuredifference between the hood 36 and the region between the fan 32 and theflow straightening element 34) and therefore determine the airflow.Pressure sensors 38 a and 38 b positioned before and after theflow-straightening element 34 determine the pressure differential acrossthe flow-straightening element 34 and therefore the airflow. Therelationship between the pressure differential and the air flow isdetermined by the type of flow-straightening element 34 employed. In oneexample, the flow-straightening element 34 is a plurality of straws, andthe air flow has a linear relationship with the pressure differential.If the flow-straightening element 34 is a perforated plate, the air flowis related to the square root of the pressure differential. However, itis to be understood that other types of flow straightening elements 34can be employed to determine the pressure differential and therefore theair flow.

[0031]FIG. 6 illustrates a graph relating airflow to DC voltage (atconstant pressure) for two different types of registers. As shown, asthe DC voltage increases, the airflow increases. In both cases, the DCvoltage to flow ratio is the same.

[0032] By employing a fan 32 with an external energy source 30 tomeasure the airflow, the fan 32 does not impede or add resistance to theairflow being measured. Additionally, by employing a flow-straighteningelement 34 to straighten the airflow, the airflow measurement is notbiased by a non-uniform or swirled flow profile. To assure that the fan32 does not influence the airflow being measured, it is also possible toautomatically control the air. The system 20 measures airflow withoutsignificant bias and with a high degree of reproducibility withoutrequiring exotic sensors, electronics or other components. The sameequipment can be used to measure the airflow and control the pressure.By controlling the energy source so as to make the pressure seen by thegrille or register 26 the same as it would be without the fan 32 inplace, the fan 32 does not impede nor augment the airflow to or from thegrille or register 26.

[0033] A control circuit 40 (shown in FIG. 1) can be employed toautomatically adjust the voltage or current supplied to the externalenergy source 30 to maintain the desired zero pressure differentialbetween the hood 36 and the room. Alternatively, the pressuredifferential can be zeroed manually.

[0034] The fan 32, a pressure transducer, and control circuitry can berun on AC or DC power. The preferred example uses direct current orbatteries.

[0035] The foregoing description is only exemplary of the principles ofthe invention. Many modifications and variations of the presentinvention are possible in light of the above teachings. The preferredembodiments of this invention have been disclosed, however, so that oneof ordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described. For thatreason the following claims should be studied to determine the truescope and content of this invention.

What is claimed is:
 1. A device for measuring an airflow comprising: afan and a flow measurement device to measure said airflow; and anexternal energy source to power said fan.
 2. The device as recited inclaim 1 wherein a pressure proximate to an airflow discharge point or anairflow intake point is substantially equal to a pressure in a room. 3.The device as recited in claim 1 wherein an RPM/flow calibration of saidfan is insensitive to pressure differentials.
 4. The device as recitedin claim 1 wherein said fan is insensitive to swirl.
 5. The device asrecited in claim 1 wherein said flow measurement device measuresrevolutions per minute of said fan to indicate a measure of saidairflow.
 6. The device as recited in claim 5 further including a flowstraightening element to straighten said airflow and wherein an RPM/flowcalibration of said fan is sensitive to a pressure differential acrosssaid fan and said flow straightening element maintains said pressuredifferential across said fan to prevent a change in said RPM/flowcalibration of more than 5%.
 7. The device as recited in claim 6 whereinsaid pressure differential across said fan is maintained between 0 and 3Pa.
 8. The device as recited in claim 5 wherein said flow measurementdevice is a tachometer.
 9. The device as recited in claim 1 wherein avoltage supplied to said external energy source indicates a measure ofsaid airflow.
 10. The device as recited in claim 1 wherein a currentsupplied to said external energy source indicates a measure of saidairflow.
 11. The device as recited in claim 1 further including a flowstraightening element to straighten said airflow, said airflow passingthrough said flow straightening element prior to passing through saidfan.
 12. The device as recited in claim 11 wherein said flowstraightening element is sized to maintain a desired pressuredifferential across said flow measurement device.
 13. The device asrecited in claim 11 wherein said flow straightening element is ahoneycomb.
 14. The device as recited in claim 11 wherein said flowstraightening element is a plurality of straws.
 15. The device asrecited in claim 11 wherein said flow straightening element is aperforated plate.
 16. The device as recited in claim 11 wherein apressure differential across said flow straightening element indicates ameasure of said airflow.
 17. The device as recited in claim 16 wherein afirst pressure sensor positioned before said flow straightening elementand a second pressure sensor positioned after said flow straighteningelement determine said pressure differential across said flowstraightening element.
 18. The device as recited in claim 1 wherein apressure differential between a room pressure and a pressure proximateto an airflow discharge or an airflow intake is automatically maintainedto reduce an impact of said airflow at said airflow discharge or saidairflow intake.
 19. The device as recited in claim 18 wherein saidpressure differential is maintained less than 1 Pa.
 20. The device asrecited in claim 18 further including a control circuit to adjust anenergy supplied to said fan to control said pressure.
 21. The device asrecited in claim 1 wherein said external energy source is one of anelectric motor and a battery.
 22. The device as recited in claim 1wherein said fan is a muffin fan.
 23. A system for measuring an airflowcomprising: an HVAC system; a duct system to supply air from said HVACsystem; a fan and a flow measurement device to measure said airflow; andan external energy source to power said fan.
 24. A method of measuringan airflow comprising the steps of: generating an airflow; powering afan with an external energy source; and measuring said airflow.
 25. Themethod as recited in claim 24 further including the step ofstraightening said airflow.
 26. The method as recited in claim 24further including the steps of maintaining a pressure downstream of anairflow discharge point which is substantially equal to a pressure in aroom.
 27. The method as recited in claim 26 wherein the step ofmeasuring said airflow includes measuring revolutions per minute of saidfan, and a RPM calibration of said fan is insensitive to a pressuredifferential across said fan.
 28. The method as recited in claim 26wherein the step of measuring said airflow includes measuringrevolutions per minute of said fan, and said fan is sensitive topressure differentials, and further including the step of straighteningsaid airflow to maintain a desired pressure differential across said fanbetween 0 Pa and 3 Pa.